In a central tower receiver power plant, an array of heliostats reflects sunlight toward a receiver mounted atop a tower. One type of receiver converts incident radiant energy into output high-pressure, high-temperature steam, which can later be fed to a turbine for electrical power generation. Heliostats are generally mounted on the ground in an area about the tower. Each heliostat has a rigid reflective surface, such as a mirror, capable of suntracking, that is, the surface takes on orientations throughout the day so as to optimally redirect sun light from the moving sun toward the receiver.
One approach is to have comparatively large heliostats but a smaller number of such heliostats. In such a power plant, the fewer number of heliostats can make it economical to manufacture very precise, and thus very expensive, components for the positioning of the reflective surfaces. Another approach, however, is to use comparatively small heliostats, such as with reflective surfaces that measure about 1 meter by 2 meters. On one hand, such an approach can be more efficient at redirecting sun light because there are more individually adjustable reflective surfaces. Such an approach can also advantageously facilitate installation because of their smaller sizes. For example, a two man crew could readily install such heliostats. On the other hand, however, more heliostats equates to more drive assemblies required for the heliostats and more steps for installation that must be repeated. Accordingly, there is a need for heliostat assemblies that are both economical to manufacture and efficient to install.
One problem with controlling the positioning of reflectors of heliostats is that movement is preferred only in a predetermined, controlled manner. This is because accurate positioning of the reflectors is necessary to maintain efficiency of the power plant. However, wind and other environmental factors can apply loads to the reflector that move the reflector away from its preferred orientation at a given point in time of tracking the sun. Manufacturing tolerances between the components of the heliostat can contribute to backlash, undesirable movement and non-linearity in the drive systems. This can result in a greater amount of variation between the predetermined and the actual reflector orientation. While springs and other devices can be used to reduce the impact of the manufacturing tolerances, the desire to reduce the costs of manufacture of the components—particularly when a comparatively large number are required due to smaller reflector sizes—limits the amount of pointing error that can be designed out of a typical heliostat drive.
Another problem with heliostat assemblies is that the reflector is relatively flexible. The reflector of the heliostat can have a size of about 1 meter in height and about 2 meters in length. The thickness, however, may only be about 3 or 4 millimeters. Due to these dimensions and its primarily glass composition, in the case where the reflector is a glass mirror, the reflector is relatively flexible. Flexing of the reflector from a substantially planar orientation is undesirable because it can reduce the efficiency of the reflector in redirecting sunlight. To provide support for the reflector and reduce flexing, a frame is attached to the back side of the reflector. The frame includes a plurality of longitudinally extending tubes and a plurality of transversely extending tubes connecting the longitudinally extending tubes. In order to attach the elevation shaft of the drive assembly to the frame of the reflector, a bracket can be attached at each end of the elevation shaft, the shaft and brackets positioned between a pair of transverse tubes of the frame, and then bolts extending parallel to the elevation shaft used to secure the brackets to the adjacent transverse tube of the frame. Such an arrangement can disadvantageously result in undesirable flexing of the frame and hence the reflector when the bolts are tightened. For example, the two transverse members to which the elevation tube is anchored, via the brackets, can be drawn toward each other, resulting in a concave, convex or twisted reflector surface.